One method for the manufacture of a polyolefin polymer uses a gas phase fluidized bed process. In the gas phase process, a catalyst used to react monomers to polymers is commonly a dry particulate (finely divided), solid catalyst. Typically, the finely divided solid catalyst is fed into a reaction vessel whereby gaseous monomers polymerize. Ideally, the catalyst should be introduced continuously in order to maintain steady state conditions in the reaction zone.
One method and device for feeding finely divided solid catalyst into a polymerization reactor is described in U.S. Pat. Nos. 3,779,712 and 3,876,602. Referring to prior art FIG. 1, these patents describe a catalyst feeder device comprising a solids reservoir 1, an agitator 3, a screen 5, a lower casing section 7, a port 9, a metering disc 11, a pickup block or entrainment chamber 13, an injection or capillary tube 15, and other details of the device. Using this device, a discrete amount of catalyst is transported into a plurality of cavities 17 in the metering disc 11 as each of the cavities 17 are exposed to catalyst in the lower casing section 7. As the metering disc rotates, the catalyst in each of the cavities 17 drops into the entrainment chamber 13 as each cavity 17 aligns with the inlet 19. A carrier gas is continuously fed into the pickup block 13 through a tangential entrance port 21. The carrier gas then carriers the catalyst through the injection or capillary tube 15 into the polymerization reactor.
Referring to prior art FIG. 2, improvements to the device described above include the additional of a cover plate assembly and wear plate 101 to provide better sealing on each side of the metering disc 103 and assure proper isolation of the pickup section (not shown) from the rest of the feeder. The cover plate assembly comprises a cover plate 105, a floating cover plate 107, and a plurality of springs 109 located between the cover plate 105 and floating cover plate 107. The cover plate 107 is bolted to the bottom forging or bottom flange 111 of the catalyst feeder. The floating cover plate 107 rests on top of the metering disc 103. The springs 109 push the floating cover plate 107 down on the metering disc 103 to effect a good seal between the catalyst chamber above and a plurality of cavities 113 in the metering disk 103.
The floating cover plate 107 typically has a layer of polyurethane glued to it on the side contacting the metering disc 103. This layer of polyurethane helps provide a seal to the top of the metering disc 103 and provides a wearing surface that will not damage the metering disc 103. The floating cover plate 107 is assembled to the cover plate 105 and is held loosely in position by two guide pins 115. This allows the floating cover plate 107 to move freely but still maintain a constant pressure on the metering disc 103.
With reference to FIG. 2, the wear plate 101 is located below the metering disc 103. The purpose of the wear plate 101 is to provide a bearing surface for the metering disc 103. The disc contact side of the wear plate 101 is typically covered with a sealing material, wherein the sealing material is typically polyurethane. The wear plate 101 is typically secured to the bottom forging or bottom flange 111 of the feeder and provides a surface for sealing the cavities in the metering disc 103 from the pickup section. This seal, in conjunction with the floating cover plate 107 prevents fine solids, such as dry catalyst powder from free flowing into the reactor.
Dry particulate catalyst feeders are typically operated at low rotation speeds. The catalyst feeder disc is rotated by a variable speed motor, typically, for example, a 0 to 1800 rpm high torque motor, through a gearbox with a turndown ratio, for example, of 900:1 to 1500:1. Rotating the metering disc at low speed may result in overheating of the motor, or stalling the motor due to the high torque required at the low rotation speed. Thus, minimum motor speeds, for example, of about 200 rpm are typically required in order to provide the torque required to turn the metering disc without overheating or stalling the motor. This means the metering disc typically rotates, for example, at a speed of about 0.13 to about 2 rpm, with about 0.13 rpm being a minimum rotation speed.
The dry particulate catalyst feeders described above are used to feed a large variety of catalyst systems. Any catalyst made in dry power form can typically be fed to the reaction system using a dry particulate catalyst feeder. The amount of catalyst fed is dependent on the volume and number of cavities in the metering disc and the speed the metering disc turns. Typically, a metering disc is selected with the proper volume and number of cavities to provide the desired catalyst feed rate over the range of rotation speeds of the metering disc. Changing the speed the metering disc rotates is then used to control the catalyst feed rate and ultimately the production rate of the polymerization reaction. Ideally, one size metering disc is used to feed all catalysts. However, with the development of modern high activity catalyst and the development of high capacity reaction systems with high turn-down ratios, it is becoming more difficult to provide a catalyst feeder that can feed enough catalyst at the highest rates on the lowest productivity catalyst and still feed the small amount of a high productivity catalyst required at a low or start-up production rate.
Due to the combination of high activity catalyst and minimum rotation speed requirements of the catalyst feeder, the turndown ratio of a reaction system may be limited. Furthermore, a catalyst feeder may have problems when attempting to feed a small amount of catalyst during a reactor start-up. To help increase the range of the volume of catalyst that can be fed, dual pickup sections may be installed on a single catalyst feeder. When one pickup section is isolated by a valve, that pickup section does not feed catalyst, giving the feeder half of the feed capacity as compared to when both pickup sections are in service. However, providing a broad range of catalyst injection rates remains a problem in the industry. Other background reference include U.S. Pat. No. 5,209,607, GB 1 295, 459, DE 195 00 726 A1, WO 02/096643, JP 09 013005, and EP 0 232 922 A.
Accordingly, there exists a need to provide a solids feeder with the ability to operate at lower rotation speeds without stalling or damaging drive motors. It is further desirable to lower the torque required to turn the solids feeder disc, which in turn will lower the torque that the drive motor must produce to turn the solids feeder disc.